Downhole tools are delivered on strings to proper positions in the wellbore. In the case of packers an inner string is used to set the packer when the proper depth is reached. It is at times desirable to set downhole tools in a particular sequence. Bottom up has been a way to set a series of external casing packers on a casing string.
One type of actuating mechanism that has been used in the past is to take advantage of hydrostatic pressure available in the wellbore to set a downhole tool like a sliding sleeve or a packer, for example. These hydrostatically actuated mechanisms were still triggered by an inner string that shifted a sleeve, for example, to allow the hydrostatic pressure access to an actuating piston. An example of the prior design of such a triggering mechanism that was actuated by hydrostatic pressure is illustrated in FIGS. 1 and 2. A mandrel 10 is illustrated schematically to have a valve 12 that can selectively align hydrostatic pressure in passage 14 to first atmospheric chamber 16. The prior art design generally used a sliding sleeve for valve 12 that had to be shifted manually. Seals 18 and 20 along with valve 12 initially hold the atmospheric pressure in chamber 16. Passage 14 can be inside the mandrel 10 or outside of it. As shown in FIG. 2, when valve 12 is opened with an inner string, for example, that is not shown, the hydrostatic pressure communicates from passage 14 to chamber 16 and winds up pushing piston as chamber 24, which is initially at atmospheric pressure as was chamber 16 is reduced in volume while chamber 16 increases in volume. The movement of piston 22 causes these volume changes. Atmospheric pressure in chamber 24 is initially trapped there by seals 26 and 28. Movement of piston 22 operates a downhole tool, like setting an external casing packer, for example.
The reason for the valve 12 is to allow time for proper positioning of the downhole tool, such as a packer, before the hydrostatic pressure sets it. Since the exact trigger depth cannot be a certainty, the past designs have employed valves such as 12 in conjunction with an inner string to operate said valves when the proper placement was assured. However, running another string to set one or more downhole tools such as packers was time consuming and therefore expensive. Additionally, once the sliding sleeve that functioned as valve 12 was actuated, there was no delay and the downhole tool set immediately from hydrostatic pressure. The present invention provides a solution to the cost of the running of the inner string by a creating a delay incorporated into the prior trigger designs that allow time to ensure proper placement before the tool is set and yet removes the need for running an inner string or the like to initiate the setting sequence. Multiple mechanisms can be employed with different delay times built in to trigger devices in a particular order. These and other advantages of the present invention will be more apparent to those skilled in the art from a review of the description of the preferred embodiment, the drawings and the claims below, which define the scope of he invention.